Today a number of different standards for telecommunication exist. One such existing telecommunication standard is Code Division Multiple Access (CDMA) another standard is termed Wideband Code Division Multiple Access (WCDMA).
In a WCDMA system, which is a powerful standard able to transmit data over a radio network at very high speed, a User Equipment (UE) such as a mobile telephone communicates over radio channels with base stations typically denoted Node B. Each base station usually transmits and receives signals over selected radio channels for a particular geographic region known as a cell. The cell can in turn be subdivided into several sectors. Typically, a number of base stations are connected to a base station controller node a radio network controller node (RNC). One or more RNCs are, in turn, connected to or included with a mobile switching center (MSC). The mobile switching center is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network or to a packet-data network such as the Internet.
In a wideband code division multiple access (WCDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are utilizing the same radio frequency. Thus, in WCDMA each mobile radio employs its own unique code sequence to encode its data signal. The receiver, knowing the code sequences of the mobile radio it services, decodes the received signal to recover data from each radio.
The WCDMA encoding process enlarges the spectrum of the signal and is therefore known as spread-spectrum modulation. In WCDMA the bandwidth of the transmitted signal can be very wide indeed. The resulting signal is also called a spread-spectrum signal. The spectral spreading of the transmitted signal gives to WCDMA its multiple access capability. That is, if multiple radios transmit a spread-spectrum signal at the same time, the receiving station will still be able to distinguish between the radios because each user has a unique code that has a sufficiently low cross-correlation with other codes used simultaneously by other radios.
Correlating the received signal with a code signal from a certain radio despreads the signal from that radio, while the other spread-spectrum signals will remain spread over a large bandwidth. Thus, after decoding a signal from a particular radio within the information bandwidth, the power of the desired radio signal will be larger than the interfering power of the other radios. With that power discrepancy, the desired signal can be extracted.
In a WCDMA system, power control is very important. In the uplink direction, the requirement for power control arises because of the possibility for multiple access interference. All radios in a cell using a WCDMA system transmit their data using the same bandwidth at the same time as other radios in that cell. Further, in a WCDMA system the neighboring cell frequencies are the same as in a given cell. Thus, interference can be seen into neighboring cells, causing capacity degradation. In such a system, it is inevitable that radios will interfere with one another. Signals received by the base station from a radio close to the base stations, for example, will be stronger than signals received from radios located at a cell boundary. Distant radios will thus tend to be dominated by close ones.
Depending on the circumstances the system from time to time may employ different handover techniques, such as soft and hard handovers. In soft handover a mobile station is connected to more than one base station simultaneously.
In WCDMA mobile communications, typically the same base band signal with suitable spreading is sent from several base stations with overlapping coverage. The mobile terminal can thus receive and use signals from several base stations simultaneously. Moreover, since the radio environment changes rapidly, a mobile station likely has radio channels to several base stations at the same moment, e.g., so that the mobile station can select the best channel and, if necessary, use signals directed to the mobile from various base stations in order to keep radio interference low and capacity high.
A general problem in most radio network systems including WCDMA is, as mentioned above, the existence of noise and interference. The presence of noise and interference will cause a number of unwanted events to occur. For example, the level of noise and interference will determine the power level required for transmitting data in a radio network. If the level of noise and interference is high the data transmission will require more power in order to be distinguishable over the background noise.
Also, a high power level for transmitting data and other signaling will in itself generate a raise in the overall level of noise and interference in the radio network, which of course is undesired.
In addition, the transmission power should be kept at a minimum to minimize power consumption. This is particularly important if the transmitting device is mobile and includes a battery that has to be recharged. A high power level will discharge batteries much more quickly.
The above applies both to CDMA systems as well as WCDMA systems.